1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus and an electrophotographic image-forming method utilizing an electrophotographic photosensitive member of high gamma, in particular, an electrophotographic photosensitive member for digital photo-input. More precisely, the present invention relates to an electrophotographic image-forming apparatus and an electrophotographic image-forming method utilizing a novel charge erasing method.
2. Background Art
As shown in FIG. 5, conventional electrophotographic image-forming apparatuses have, for example, a structure comprising, around a rotatably maintained cylindrical photosensitive member 1, a main electrifier 2, light exposure means 3, image development means 4, image transfer means 5, cleaning means 6, and charge erasing (eliminating) light source 7 in this order, and charge erasing of the photosensitive member is attained by light-exposing the surface of the photosensitive member 1 by means of the charge erasing light source 7.
On the other hand, photosensitive members of high gamma have been developed in recent years in order to form static latent images with high resolution in electrophotographic image formation, in particular, by digital photo-input. For example, Japanese Patent Publication (Kokoku) No. 5-19140 discloses an electrophotographic photosensitive member for digital photo-input having a gamma for latent image of 6 or more, which comprises a photosensitive layer consisting of intrinsic semiconductor fine powder contained in a binder having a high volume resistivity. Japanese Patent Unexamined Publication (Kokai) No. 8-286396 also discloses a photosensitive member having a gamma value of 2 to less than 6.
However, because such photosensitive layers of high gamma utilize a highly resistive binder, charge transfer therein is strongly prevented, and after electrification, a part of charge flowing in the photosensitive members by the light exposure remains throughout the whole photosensitive layer. Therefore, it is difficult to completely erase the charge after the image formation only by the light exposure as in the conventional electrophotographic image-forming apparatuses. As a result, the high-gamma photosensitive members suffer a problem of so-called hysteresis, i.e., electrification potential may be different among light-exposed areas and unexposed areas in a former image formation process, and thus images of high resolution cannot be stably formed.
Moreover, there is also the problem of sensitivity shift, i.e., shift of the sensitivity curve of the photosensitive member in the higher sensitivity direction due to the residual charge in the photosensitive member caused by repetition of image formation.
A larger light exposure amount for charge erasing may excite a deeper portion of the photosensitive member by its photocurrent, thereby providing uniform charge distribution. However, the light energy may accelerate yield of the binder of the photosensitive member, and hence dark decay may become faster. This may make sequential image-forming operations impossible.
Furthermore, reversal development is usually used in printers, in which a transfer electrifier is applied with a voltage of opposite polarity to that applied to the main electrifier, and may invite another problem. That is, such voltage of reverse polarity provides a field of opposite polarity to that of the main electrification inside the photosensitive member, and this may cause sensitivity variation due to higher internal field during the main electrification. Thus, images cannot be stably formed.
Therefore, various techniques have been proposed in order to solve such problems as mentioned above. For example, Japanese Patent Unexamined Publication (Kokai) No. 4-337762 proposes a method comprising controlling light exposure amounts for image formation and charge erasing to be in a particular relationship in order to solve the aforementioned problem of hysteresis. This method may achieve the desired effect for a photosensitive member of a gamma for latent image of less than 3. However, when the gamma for latent image is 3 or higher, completely stable operation cannot be performed even with this technique, because of sensitivity variation due to ununiform electrification after repeated image formation and the like.
Therefore, the object of the present invention is to provide an image-forming apparatus and an image-forming method that can solve the aforementioned problems, specifically an apparatus and a method enabling effective uniform charge erasing (eliminating), hence not causing the so-called hysteresis or sensitivity shift even in continuous image formation, and capable of stably forming images of high quality and high resolution without residual images in electrophotographic image formation utilizing a high-gamma photosensitive member.